A pretensioner for a seat belt system is disclosed in U.S. Pat. No. 5,588,677, which is assigned to the assignee of this invention, and in this pretensioner a buckle part of a three point seat belt system is connected to a flexible draw element in the form of an elongated wire cable. The wire cable has a loop portion looped about a roller secured to the buckle part. One end of the loop portion is secured and the other portion of the loop is pulled downwardly by a power-operated device. The preferred power-operated device has a piston which travels horizontally and pulls the attached horizontal cable portion horizontally which in turn pulls down the other portion of the loop and the attached buckle part. The loop splits the force being applied to the buckle in half, e.g., about 2,600 lbs. on each half of the loop but the loop requires twice the travel distances for the cable for each increment of downward buckle travel.
Typically, the power-operated device is a pyrotechnic gas generator that drives a piston to travel the distance necessary to remove the slack from the seat belt system. Such cylinders are usually located on or about the seat. In some instances, there is a lack of sufficient space for such a long cylinder, and there is a desire to provide a more compact pretensioner of this kind.
When the piston has traveled the full extent of the cylinder and has taken out the slack, the pretensioner will have pulled the person down into the seat, and thereafter the person and attached seat belt system begin to rebound upwardly. To prevent the passenger from rebounding and returning slack into seat belt system, a one-way locking mechanism becomes effective to resist the rebound travel of the cable. Such an automatic locking of the return travel of the wire cable is disclosed in German published application DE 42 30 663 published Mar. 17, 1994.
The pulling force of the pretensioner on the buckle part is quite large, and where the buckle part is a buckle having a locking mechanism operated by a push button, it is preferred to have a latch or blocking member that is shifted to a blocking position to prevent the push button from traveling to release the latch plate. European published patent application 0 557 073, published Jun. 29, 1993, discloses a pivotally mounted blocking member that is biased by a spring to a normal, non-blocking position and that is pivoted against the spring bias by the gas generator to a blocking position. In the blocking position, the blocking member engages and blocks travel of the push button member in the release direction. Thus, the gas generator causes a blocking of the buckle release at the time that the pretensioner is removing slack from the seat belt system.
When the acceleration sensor activates the pretensioner, it activates both the driver's pretensioner as well as the passenger's pretensioner. If the driver or the passenger is not wearing the seat belt system at the time of actuation of the pretensioner, the buckle travels downwardly very rapidly and hits its buckle housing so hard against the pretensioner housing that the buckle housing can crack or break. In order to avoid a liability claim, where a person not wearing the seat belt system alleges to have been wearing it and alleges that the seat belt buckle failed as evidenced by the broken buckle housing, it would be desirable to provide an energy absorption device or member to absorb energy and dampen the buckle impact such that the buckle housing doesn't break. Thus, a potential liability claim could be avoided.